120 SEXUAL SELECTION: BIRDS. [Part II. 



insensibly graduate into each other, namely, slight differ- 

 ences between all the members of the same species, and 

 more strongly-marked deviations which occur only occa- 

 sionally. These latter are rare with birds in a state of 

 nature, and it is very doubtful whether they have often 

 been preserved through selection, and then transmitted to 

 succeeding generations. 32 Nevertheless, it may be worth 

 while to give the few cases relating chiefly to color (sim- 

 ple albinism and melanism being excluded), which I have 

 been able to collect. 



Mr. Gould is well known rarely to admit the existence 

 of varieties, for he esteems very slight differences as spe- 

 cific ; now he states 33 that near Bogota certain humming- 

 birds belonging to the genus Cynanthus are divided into 

 two or three races or varieties, which differ from each 

 other in the coloring of the tail — " some having the whole 

 of the feathers blue, while others have the eight central 

 ones tipped with beautiful green." It does not appear 

 that intermediate gradations have been observed in this 



birds. It is also an unsettled point with naturalists, whether several 

 North American birds ought to be ranked as specifically distinct from 

 the corresponding European species. 



32 'Origin of Species,' fifth edit. 1869, p. 104. I had always per- 

 ceived that rare and strongly-marked deviations of structure, deserving 

 to be called monstrosities, could seldom be preserved through natural 

 selection, and that the preservation of even highly-beneficial variations 

 would depend to a certain extent on chance. I had also fully appre- 

 ciated the importance of mere individual differences, and this led me to 

 insist so strongly on the importance of that unconscious form of selection 

 by- man, which follows from the preservation of the most valued individ- 

 uals of each breed, without any intention on his part to modify the char- 

 acters of the breed. But until I read^an able article in the ' North Brit- 

 ish Review ' (March, .1867, p. 280, et seq.), which has been of more use to 

 me than any other Review, I did not see how great the chances were 

 against the preservation of variations, whether slight or strongly pro- 

 nounced, occurring only in single individuals. 



33 « Introduct. to the Trochilidse,' p. 102. 



